It is known to use fluids such as water and oils to transfer heat away from thermal systems. However, the capacity of a fluid to store and to later release thermal energy, known as specific heat or Cp, is limited to the range of approximately 0.2 to 1.0 BTU/Lb..degree.F. or kj/kg .degree.C.
Increasing demands have been required of heat transfer fluids due to high temperature applications such as lasers, electronic systems and nuclear energy, where potentially large quantities of energy must be removed from a system in short periods of time, or in space applications, where minimizing pumping energy and overall system size or weight are the primary considerations.
As a result of the foregoing, efforts have been made to enhance the thermal energy storage and transfer capabilities of heat transfer fluids. One such attempt was made by Mehalick and Tweedy of General Electric. Their concept was a system composed of an encapsulated phase change material (PCM) slurried in water to comprise a water-heat transfer medium. Paraffins were chosen because of their congruent-melting behavior, wide range of melting points, low cost, and low supercooling. The paraffin was coated with either gelatin or a modified nylon in particle diameters from less than 50 microns to 2000 microns. The weight of the coating was from 10 to 25 per cent of the total particle weight. The best slurry contained 40 per cent by weight of the encapsulated PCM in water. This system claims to have had a heat storage capacity twice that of water under an 11 degree centigrade temperature change and had a heat-transfer rate within 70 to 80% of the transfer rate of a water system under similar conditions. The research was terminated before amarketable product was developed because of failure of the microcapsules in a pumpable slurry. (See Mehalick, E.M. and Tweedie, A.T., TWO-COMPONENT THERMAL ENERGY STORAGE MATERIAL, PROC. WORKSHOP SOL. ENERGY STORAGE SUBSYSTEMS FOR THE HEATING AND COOLING OF BUILDINGS, Lilleleht. .J., Ed., University Press of Virginia, Charlottesville, 1975, p. 85 and Mehalick, E.M. and Tweedie, A.T., TWO-COMPONENT THERMAL STORAGE MATERIAL STUDY, PHASE II, Report COO-2845-78/2, U.S. Department of Energy Washington, D.C., 1979.)
An idea similar to that of Mehalick and Tweedie is disclosed in French Patent WO 81/02163 to Naizot which discloses an energy-storage material contained in a metallic shell to form spherical particles with diameters between 10 and 1000 microns. These microcapsules were suspended in a liquid heat transfer fluid. The result was a heat-transfer fluid with improved heat-storage capacity. The aforementioned prior art systems are not without their limitations as they require a compromise to be made between PCM concentration (which enhances thermal energy storage as the concentration increases) and the pumpability of the slurry (which decreases as PCM concentration increases). Thus, while the prior art systems offered an improved thermal capacity they were nevertheless limited in usefulness and never achieved commercial success.
In view of the foregoing, it is an object of the present invention to provide a method of employing a PCM slurry that offers a significantly higher thermal storage and transport capacity than is available with current technology.
Another object of the present invention is to provide a method of employing a PCM slurry that significantly improves the heat transfer coefficient between the fluid and wall of a heat exchanger.
A further object of the present invention is to provide a method of employing a slurry that requires reduced pumping energy in order to be pumped for a given quantity of energy to be transported.
A still further object of the present invention is to provide a method of employing a slurry that reduces the size and weight of a thermal system required to cool a heat source.